1) Field of the Invention
This invention relates to a brake band suitable for use in a transmission or like of a vehicle, for example, an automobile, an agricultural machinery or a construction machinery.
2) Description of the Related Art
In an automatic transmission, multiplate clutches, brake bands and the like are used to effect speed changes by causing an input element such as a gear train to engage or releasing the same. Of these, brake bands are widely used for their preferred characteristic that, even if the same application is applied to a drum, their braking force differs depending on the braking direction of the drum and smooth up-shifting is feasible.
A fundamental construction of a brake band is illustrated in FIG. 15. The brake band, which is generally indicated at numeral 7, is formed of a curved strap 3 made of a thin steel plate. Brackets 4,4 are connected to opposite end portions of the strap 3, respectively. A porous thin friction material (lining) 2 is bonded with an adhesive on an inner peripheral surface of the strap 3.
Of these brackets, the bracket 4 is an apply-side bracket on which hydraulic pressure for actuating the brake acts, whereas the bracket 4' is a anchor-side bracket.
To improve the oil discharge characteristic between the drum and the lining in the course of a binding operation of the brake band in such a band brake, the lining 2 may be provided with an oil groove 5, which may in turn be provided with oil discharging apertures 6.
The action of the brake band will next be described in short with reference to FIG. 16. The brake band encircles a drum 1. When it is desired to actuate the brake, hydraulic pressure is applied as indicated by arrow P to the bracket 4 on an apply side to bind the brake band. Designated at 4' is the anchor-side bracket on an opposite end, that is, on a support side.
When the drum is rotating in a direction indicated by arrow L, the band brake is used in a so-called "leading state". When the drum is rotating in an opposite direction, the band brake is then used in a so-called "trailing state".
Unless the oil groove 5 and the oil discharging apertures 6 are arranged appropriately, the discharging function of the brake band for an oil film existing between the drum and the lining in the course of binding of the brake band is low and the brake band exhibits a low friction characteristic especially during an initial period of binding in which the pressing force is low. This leads to a sudden increase in binding force at a final stage of binding, resulting in an increased shift shock.
It was hence proposed to provide a brake lining with such oil grooves and oil discharging apertures as shown in FIGS. 8 and 9. In these drawings, the oil grooves and the oil discharging apertures are indicated by numerals 8 and 9, respectively. Designated at numeral 8' are oil grooves communicating to an exterior at opposite ends of the brake linings.
In each of FIGS. 8 and 9, the oil grooves and oil discharging apertures are arranged symmetrically relative to a central line which as viewed in the drawing, vertically extends at a right angle with respect to a longitudinal central axis of the brake lining, in other words, on an apply side and an anchor side.
In the lining 2a shown in FIG. 8, the oil grooves 8, which are each provided with the oil discharging aperture 9, are arranged in three rows. In the lining 2b illustrated in FIG. 9, on the other hand, the oil grooves 8, each of which is also provided with the oil discharging aperture 9 are arranged in a single row along the longitudinal central axis.
Various potential inconveniences may however arise if oil grooves and oil discharging apertures are arranged, as shown in FIGS. 8 and 9, over an entire area of a brake lining from its apply side to its anchor side and symmetrically relative to a central line extending at a right angle with respect to a longitudinal central axis of the brake lining.
In the case of the brake lining 2a depicted in FIG. 8, the discharge of oil is excessive so that the starting friction torque is large and the shifting performance is deteriorated. In the case of the brake lining 2b illustrated in FIG. 9, the discharge of oil is insufficient so that the starting friction torque is small and the binding force suddenly increases at a final stage of binding. The brake linings 2a, 2b are unavoidably accompanied by these drawbacks.
To overcome such drawbacks, it has therefore been proposed to make the total area of oil discharging means, which are provided in the portion of a brake lining which is near an apply side end of a brake band, greater than that of oil discharging means in the portion of the brake lining which is on an anchor side of the brake band as shown in U.S. Pat. No. 5,135,082 which has been assigned to the same assignee as the present invention and is hereby incorporated herein by reference.
When such a brake band is used, its torque curve becomes flat as shown in FIG. 12 so that shift shocks can be reduced. When a drum is rotating in a leading direction, however, a surface pressure applied on the anchor side end is generally at least twice as much as a surface pressure applied on the apply side. More heat is therefore produced on the anchor side and especially when the brake band is used at elevated temperatures, in other words, in the state of low oil viscosity, thereby making it impossible to maintain an oil film on the anchor side. The brake lining is therefore caused to burn out little by little from its surface, whereby a greater torque change takes place with time. In the middle of its use, this torque change can be illustrated as shown in FIG. 13. At the end of its use, it can be depicted as shown in FIG. 14. As is understood from the changes in torque curve shown in FIGS. 13 and 14, the torque at the beginning of each engagement and hence, the shift shock increase as the time of use becomes longer.